1) Field of the Invention
The present invention relates to an improvement of security of polymer electrolyte secondary cells.
2) Description of the Related Art
In recent years, an increasing need for thin, light-weight cells or batteries with high performance has prompted use of non-aqueous electrolyte secondary cells with polymer electrolytes (also referred to as polymer electrolyte secondary cells). Provided with high energy density and difficulty for solution leakage, polymer electrolyte secondary cells are suitable for the power sources of mobile appliances. However, the polymer electrolyte secondary cells are inferior in electrical characteristics such as a discharge characteristic to usual non-aqueous electrolyte secondary cells with liquid non-aqueous electrolytes.
Here is a possible explanation for the drawback. In the polymer electrolyte, a polymer matrix resulting from polymerization of a polymer precursor (monomer) holds a liquid non-aqueous electrolyte. Since the polymer matrix itself has no ion conductivity, an increase in the polymer in the polymer electrolyte hinders conductivity for lithium ions, resulting in a reduction in discharge characteristic of the cell. Meanwhile, a reduction in the polymer in the polymer electrolyte wanes the capability to hold the liquid non-aqueous electrolyte, resulting in solution leakage.
Thus, prevention of solution leakage and the improvement in discharge characteristic are in a trade-off relationship, making it difficult to secure both of them at the same time.
In addition, the polymer electrolyte uses a flammable organic solvent, which poses a possibility of smoking, fire, and the like when the cell is overcharged. Thus, there is a need for security during overcharge of the cell.
Technologies concerning the non-aqueous electrolyte secondary cell are proposed in Japanese Patent Application Publication No. 2004-63385 (patent document 1), WO02-015319 (patent document 2), Japanese Patent Application Publication No. 2003-297422 (patent document 3), Japanese Patent Application Publication No. 2003-59529 (patent document 4), Japanese Patent Application Publication No. 2004-63385 (patent document 5), Japanese Patent Application Publication No. 2006-278260 (patent document 6), Japanese Patent Application Publication No. 2006-32301 (patent document 7), Japanese Patent Application Publication No. 10-116516 (patent document 8), and Japanese Patent Application Publication No. 10-283839 (patent document 9).
In patent document 1, a polymer formed of alkylene glycol di(meth)acrylate represented by Formula 1 serves as a matrix and holds therein electrolyte salt and a solvent for the electrolyte salt.

wherein R1 denotes an alkylene group with 2 to 12 carbon atoms, and R2 denotes a hydrogen atom or a methyl group.
This technology, however, requires much polymer; the compound needs to be approximately 0.5 g (approximately 3.2 mass %) relative to 15 g for the electrolytic solution, in order to obtain a preferable polymer electrolyte. This poses the problem of an insufficient discharge characteristic.
In patent document 2, an electrolytic salt containing fluorine and tertiary carboxylic acid ester are contained in a non-aqueous solvent containing cyclic carbonate, in order to obtain a cell with superior cell characteristics such as a cycle characteristic.
This technology, however, is related to a non-aqueous electrolyte secondary cell and therefore cannot be applied as it is to a polymer electrolyte secondary cell.
In patent document 3, carboxylic ester and carboxylate ion are contained in the electrolyte, in order to obtain a cell with superior cell characteristics such as the cycle characteristic.
This technology, however, is related to a non-aqueous electrolyte secondary cell and therefore cannot be applied as it is to a polymer electrolyte secondary cell.
In patent document 4, cyclic carbonate and/or cyclic ester, an alkyne derivative, and tertiary carboxylic ester are contained in a non-aqueous solvent, in order to obtain a cell with superior cell characteristics such as the cycle characteristic.
This technology, however, is related to a non-aqueous electrolyte secondary cell and therefore cannot be applied as it is to a polymer electrolyte secondary cell.
In patent document 5, a cross-linked polymer formed of at least one multi-functional (meth)acrylate selected from multi-functional (meth)acrylate represented by Formula 2 and multi-functional (meth)acrylate represented by Formula 3 serves as a polymer matrix and holds therein electrolytic salt and a solvent for the electrolytic salt, thereby forming a gel electrolyte with high performance.

This technology, however, requires much polymer; the compound needs to be approximately 0.5 g (approximately 3.2 mass %) relative to 15 g for the electrolytic solution, in order to obtain a preferable polymer electrolyte. This poses the problem of insufficient discharge characteristic.
In patent document 6, a non-aqueous electrolytic solution contains a halogen-substituted organic compound and a polymerizable monomer such as a vinyl monomer, in order to obtain a cell with enhanced security in case of thermal runaway of the cell.
This technology, however, is related to a non-aqueous electrolyte secondary cell and therefore cannot be applied as it is to a polymer electrolyte secondary cell.
In patent document 7, the electrolyte contains a solvent containing at least one selected from the group consisting of carboxylic acid represented by Formula 4 and ketone represented by Formula 5, in order to obtain a cell with a superior cycle characteristic.

This technology still provides an insufficient discharge characteristic.
Patent document 8 is related to a cell with a polymer solid electrolyte containing: a polymer matrix formed by copolymerization of a polymerizable monomer represented by Structural Formula (I) and a cross-linking agent represented by Structural Formula (II); a polymerization initiator; and an electrolytic solution made of an inorganic salt and a solvent, in order to obtain a solid electrolyte secondary cell superior in ion conductivity.

This technology, however, requires much polymer; the compound needs to be approximately 4 g (approximately 20 mass %) relative to 15.8 g for the electrolytic solution, in order to obtain a preferable polymer electrolyte. This poses the problem of an insufficient discharge characteristic.
Patent document 9 is related to a polymer solid electrolyte having vinylidene fluoride resin and/or N,N-diethyl acrylamide contained in a polymer electrolyte solvent containing: a polymer matrix formed by copolymerization of a polymerizable monomer represented by Structural Formula (1) and a cross-linking agent represented by Structural Formula (2); a polymerization initiator; and an electrolytic solution made of an inorganic salt and a solvent, in order to obtain a solid electrolyte secondary cell superior in ion conductivity and mechanical strength,

This technology still provides an insufficient discharge characteristic.